Impact of including exiting vehicles in single-lane roundabout capacity models

Roundabouts are classified as alternative intersection forms because they provide operational conditions not found at conventional intersections (e.g., a low-speed environment and fewer possible vehicle conflicts). Such conditions provide a combination of safety and operational benefit that normally is not achievable at a conventional intersection. The ability to predict capacity accurately is important when alternative intersection designs are examined. Highway Capacity Manual 2010 provides a current method for determining the capacity of a roundabout approach in which vehicles that exit on the same approach are considered not to influence capacity. However, the inclusion of exiting vehicles in the model creates a different circulating vehicle gap distribution (dividing larger gaps into smaller gaps) that reduces the number of opportunities to measure follow-up headway. Also, most critical headway values for the analysis are smaller than the critical headway values found when exiting vehicles are excluded. Capacity equations were developed from video imagery recorded for 28 approaches at 13 roundabouts, and the impact of including exiting vehicles in the analysis of single-lane roundabout capacity was evaluated. Vehicles exiting the roundabout were found to have a measurable impact on the estimated capacity of the roundabout approach. Both follow-up and critical headway values decreased when exiting vehicles were included. Also, the estimated capacity including exiting vehicles was found to increase or decrease (relative to capacity estimates that excluded exiting vehicles) depending on the percentage of conflicting vehicles that were exiting vehicles.